Systems and methods for measuring characteristics of cryogenic electronic devices

ABSTRACT

This disclosure relates to systems and methods for measuring impedance characteristics of a cryogenic device under test (DUT). A channel select circuit can be configured in a first state to electrically isolate a channel output circuit from the cryogenic DUT and in a second state to electrically couple the channel output circuit to the cryogenic DUT, and at least one resistor can be positioned along a transmission path that couples a pattern generator circuit to a channel output circuit that includes the channel select circuit. A controller can be configured to cause respective test current signals to be provided along the transmission path when the channel select circuit is in respective first and second states to establish respective first and second voltages across the at least one resistor, determine first and second impedance characteristics of the transmission path for determining an impedance of the cryogenic DUT.

TECHNICAL FIELD

This disclosure relates to cryogenic electronic device testing.

BACKGROUND

Cryogenic electronics refers to an operation of electronic devices,circuits, and systems at cryogenic temperatures. Cryogenic electronics(also referred to as low-temperature electronics, or cold electronics)can be based on semiconductive devices, superconductive devices, or acombination of the two.

SUMMARY

In an example, a system can include a channel select circuit that can beconfigured in a first state to electrically isolate a channel outputcircuit from a cryogenic device under test (DUT) and in a second stateto electrically couple the channel output circuit to the cryogenic DUT,at least one resistor positioned along a transmission path that couplesa pattern generator circuit to the channel output circuit that includesthe channel select circuit and a controller. The controller can beconfigured to cause respective test current signals to be provided bythe pattern generator circuit along the transmission path when thechannel select circuit is in respective first and second states toestablish respective first and second voltages across the at least oneresistor, determine first and second impedance characteristics of thetransmission path based on the established respective first and secondvoltages and an amount of current being provided by respective testcurrent signals, and determine an impedance of the cryogenic DUT basedon the first and second determined impedance characteristics of thetransmission path.

In another example, a system can include a measuring circuit that can beconfigured to measure a first voltage established across a resistoralong a transmission path based on a first current signal. The firstcurrent test signal can be provided along the transmission path when achannel output circuit is isolated from a cryogenic DUT. Thetransmission path can couple a pattern generator circuit for generatingcurrent signals to a channel output circuit. The measuring circuit canbe configured to measure a second voltage established across theresistor based on a second current signal. The second current testsignal can be provided along the transmission path when the channeloutput circuit is electrically coupled to cryogenic DUT. The measuringcircuit can be configured to determine first and second average voltagesbased on the measurements of the first and second voltages,respectively. The system includes a controller that can be configured todetermine first and second impedance characteristics of the transmissionpath based on respective first and second average voltages and an amountof current being provided by respective first and second current testsignals, and determine an impedance of the cryogenic DUT based on thefirst and second determined impedance characteristics of thetransmission path.

In a further example, a method can include causing a channel outputcircuit to be electrically isolated from a cryogenic DUT, controlling atest pattern generator circuit to provide a first current test signalalong a transmission path so that the first current test signal flowsthrough at least one resistor located in the transmission path toestablish a first voltage across the at least one resistor, receiving afirst determined average voltage based on measurements of the firstvoltage across the at least one resistor, determining first impedancecharacteristics of the transmission path based on the determined firstaverage voltage and an amount of current being provided by the firstcurrent test signal, causing the channel output circuit to beelectrically coupled to the cryogenic DUT such that the cryogenic DUT ispart of the transmission path in response to determining the firstimpedance characteristics of the transmission path, controlling the testpattern generator circuit to provide a second current test signal alongthe transmission path so that the second current test signal flowsthrough the cryogenic DUT and the at least one resistor to establish afirst voltage across the at least one resistor, receiving a seconddetermined average voltage based on measurements of the second voltageacross the at least one resistor, determining second impedancecharacteristics of the transmission path based on the second averagevoltage and an amount of current being provided by the second currenttest signal, and determining impedance characteristics of the cryogenicDUT based on the first and second impedance characteristic of thetransmission path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a cryogenic DUT testing system.

FIG. 2 is an example of another cryogenic DUT testing system.

FIG. 3 is an example of a pattern generator circuit.

FIG. 4 is an example of a method for determining impedancecharacteristics of a cryogenic DUT.

FIG. 5 is an example of another method for determining impedancecharacteristics of a cryogenic DUT.

DETAILED DESCRIPTION

For testing of cryogenic electronics, a cryogenic DUT is connected viatransmission paths (e.g., wires, conductive traces, etc.) to a testpattern generator. To test or verify a performance or characteristics ofthe cryogenic DUT, test patterns are generated by the test patterngenerator and provided via the transmission paths to the cryogenic DUT.Nonidealities and transmission mismatching, and noise can limit aperformance of a cryogenic DUT testing system, such that it becomesdifficult to accurately measure impedance characteristics of thecryogenic DUT.

Examples are described for measuring impedance characteristics of acryogenic DUT. The term “impedance” as used herein refers to a phaseshift or difference in a voltage or current operating angle. By way ofexample, the cryogenic DUT can be a Josephson Junction (JJ) device,however, in other examples, the cryogenic DUT can be another type ofcryogenic device. In the examples herein, a cryogenic DUT testing systemis configured to determine the impedance characteristics of the DUT byreducing or eliminating sources of DUT impedance error such as resultingfrom non-idealities in transmission paths and impedance mismatchesbetween the transmission paths.

For example, to determine the impedance characteristics of the cryogenicDUT, a controller of the cryogenic DUT testing system can be configuredto control a current test pattern generator circuit of the system tooutput a current test signal to a channel output circuit of the system.The controller can be configured to cause the channel output circuit tobe electrically isolated from the cryogenic DUT. By electricallyisolating the channel output circuit from the cryogenic DUT impedancecharacteristics of transmission paths of the system can be estimated ordetermined according to the examples herein. The transmission paths canbe used to electrically couple the current test pattern generatorcircuit to the channel output circuit (or circuitry therein). Thechannel output circuit can include a resistor that can be positioned orlocated along or within a respective transmission path over whichcurrent test signals can be provided to the cryogenic DUT (e.g., whenthe channel output circuit is electrically coupled to the cryogenicDUT). In some instances, the resistor can be positioned along or withinthe respective transmission path between the current test patterngenerator circuit and the channel output circuit.

The cryogenic DUT testing system includes a voltage sensing circuit anda voltage calculator circuit. The voltage sensing circuit can sense(e.g., measure) a voltage across the resistor to provide a sensedvoltage signal. The voltage calculator circuit can be configured tosample the sensed voltage signal based on a sampling rate, which can beset by the controller. The voltage calculator circuit can be configuredto determine an average voltage based on the samples of the sensedvoltage signal. The voltage calculator circuit can be configured tooutput an average voltage signal indicative of the average voltage ofthe sensed voltage signal.

The controller can be configured to determine transmission pathcharacteristics (e.g., an impedance) for the transmission paths based onthe average voltage signal and an amount of current being outputted bythe current test pattern generator circuit. Because a resistance of theresistor and a voltage across the resistor is known, and the amount ofcurrent being outputted by the current test pattern generator circuit isalso known, the controller can be configured to determine firstimpedance characteristics of the transmission paths (e.g., while thecryogenic DUT is electrically isolated from the channel output circuitand thus the transmission paths). The first impedance characteristics ofthe transmission paths can be representative of alternating current (AC)or direct current (DC) impedance characteristics of the transmissionpaths based on whether the current test pattern generator circuitoutputs an AC or DC current test signal.

In some examples, the controller can be configured to cause the channeloutput circuit to be in electrical connection with the cryogenic DUT.The impedance characteristics of transmission paths with the DUT inelectrical connection (e.g., not electrically isolated from the channeloutput circuit and thus the transmission paths) can be calculated in asame or similar manner as described herein for calculating the firstimpedance characteristics of the transmission paths. Thus, thecontroller can be configured to determine second impedancecharacteristics of the transmission paths during which the cryogenic DUTis electrically connected to the channel output circuit. The controllercan be configured to determine an impedance difference based on thefirst and second impedance characteristics of the transmission paths.The impedance difference can be representative of impedancecharacteristics (e.g., an impedance, such as an AC or DC impedance) ofthe cryogenic DUT.

Accordingly, the cryogenic DUT testing system described herein canaccurately measure impedance characteristics of the cryogenic DUT (e.g.,such as AC and/or DC impedance characteristics) by compensating fortransmission path nonidealities and impedance mismatches between thetransmission paths.

FIG. 1 is an example of a cryogenic DUT testing system 100. The system100 includes a controller 102 and a current test pattern generatorcircuit 104. The controller 102 can include a processor 106 (e.g., acentral processing unit (CPU)) and memory 108. The memory 108 canrepresent a non-transitory machine-readable memory (or other medium)that can be accessed by the processor 106 to execute impedance testlogic 110 for testing impedance characteristics (e.g., AC and/or DCimpedance characteristics) of a cryogenic DUT 112. During impedancecharacteristic testing of the cryogenic DUT 112, the processor 106 canexecute an impedance characteristic calculator 114 that can beprogrammed to determine the impedance characteristics of the cryogenicDUT 112 based on different impedance characteristics of the transmissionpaths as described herein.

The current test pattern generator circuit 104 can generate one or moredifferent test patterns. For example, the current test pattern generatorcircuit 104 can generate a differential current test signal 116 that cancorrespond to a respective test pattern or be part of the respectivetest pattern. Thus, the current test pattern generator circuit 104 insome instances can be implemented as a differential current source. Inadditional or alternative examples, the current test pattern generatorcircuit 104 can be implemented as a bipolar current source such thatcurrent can be sinked or sourced with respect to the cryogenic DUT 112.The current test pattern generator circuit 104 has a first output 118and a second output 120. In some instances, the first output 118provides a current test signal 122 and the second output 120 receivesthe current test signal 122. Thus, the first output 118 can output thecurrent test signal 122 and the second output 120 can pull the currenttest signal 122. Because the system 100 is for testing cryogenic DUTs,the current test signal 122 can be referred to as a high-precision testcurrent. A “high-precision” test current is a current signal in arelatively low current range, such as a micro-amperes or nano-amperesrange. In some instances, the current test signal 122 is an AC currenttest signal and in other instances, the current test signal 122 is a DCcurrent test signal. The AC and DC current test signals can be used todetermine AC and DC impedance characteristics of transmission paths andthe cryogenic DUT 112 as described herein.

The system 100 can further include a channel output circuit 124. Thechannel output circuit 124 is coupled between the current test patterngenerator circuit 104 and the cryogenic DUT 112. The system 100 caninclude any number of channel output circuits similar to the channeloutput circuit 124, as shown in FIG. 1 . A number of channel outputcircuits can be based on a number of cryogenic DUTs that are to besimultaneously or sequentially tested by the system 100. Thus, eachchannel output circuit can be coupled to a respective current testpattern generator circuit for receiving a corresponding test pattern(e.g., one or more differential current test signals). In otherexamples, the current test pattern generator circuit 104 can beconfigured to provide a respective test pattern to each channel outputcircuit. Each channel output circuit can be coupled to a respectivemeasuring circuit, such as a measuring circuit 126, as shown in FIG. 1 .

In the example of FIG. 1 , a single channel is shown for providing testpatterns for testing a single cryogenic DUT. While the example of FIG. 1shows a single channel, in other examples, any number of channels (e.g.,twelve (12) channels) can be used for testing a corresponding number ofcryogenic DUTs. The cryogenic DUTs, and thus the cryogenic DUT 112 canbe placed in a container, cooler, fridge, or chamber that can beconfigured to support a cryogenic environment. Each channel can beenabled or activated during cryogenic testing of the correspondingcryogenic DUT to provide a respective test pattern to the correspondingcryogenic DUT.

By way of further example, the processor 106 can execute the impedancetest logic 110 to provide a reference signal 128. The reference signal128 can directly or indirectly indicate an amount of current that is tobe outputted by the current test pattern generator circuit 104. Forexample, the reference signal 128 can indicate a voltage reference andthe current test pattern generator circuit can includevoltage-to-current (VI) converter that can provide the current testsignal 122 based on the reference signal 128. The current test patterngenerator circuit 104 can provide the differential current test signal116, which includes the current test signal 122, based on the referencesignal 128. The system 100 includes a current sensing circuit 130 tomeasure an amount of current being outputted by the current test patterngenerator circuit 104. The current sensing circuit 130 can sense (e.g.,sample) the current test signal 122 to provide a sensed current testsignal 132. In some examples, the current test pattern generator circuit104 can communicate to the controller 102 the amount of current beingoutputted as the current test signal 122.

The controller 102 can execute the impedance test logic 110 to provide achannel control signal 134. The channel control signal 134 can controlan electrical coupling of the channel output circuit 124 to thecryogenic DUT 112. The channel control signal 134 can be provided to achannel select circuit 136 to electrically isolate the channel outputcircuit 124 from the cryogenic DUT 112. By electrically isolating thechannel output circuit 124 from the cryogenic DUT 112 impedancecharacteristics of transmission paths can be estimated or determinedaccording to the examples described herein.

In some instances, the channel select circuit 136 is configured in afirst state (e.g., DUT isolating state) in which the channel outputcircuit 124 is electrically isolated from the cryogenic DUT 112. Inexamples wherein the channel select circuit 136 is in the first state,the controller 102 can provide the channel control signal 134 to switchthe channel select circuit 124 to a second state, thereby electricallycoupling the channel output circuit 124 to the cryogenic DUT 112. Insome instances, the channel select circuit 136 is implemented as asingle pole double throw relay (SPDT).

As described herein, by electrically isolating the channel outputcircuit 124 from the cryogenic DUT 112 impedance characteristics of thetransmission paths can be estimated or determined. The term“transmission path” as used herein can refer to a signal path (e.g., awire, a trace, intermediate circuitry, and/or a combination thereof)along which a test signal or pattern can be provided. In the example ofFIG. 1 , a first transmission path 138 can extend from the first output118 of the current test pattern generator circuit 104 to a first input140 of the channel select circuit 136. A second transmission path 142can extend from the second output 120 of the current test patterngenerator circuit 104 through a resistor 144 to a second input 146 ofthe channel select circuit 136, as shown in FIG. 1 . In some instances,the first and second inputs 140 and 146 correspond to the inputs of thechannel output circuit 124 and in these examples, the resistor 144 canbe located between the current test pattern generator circuit 104 andthe channel output circuit 124. Each channel output circuit can includea resistor that is positive along a respective transmission path thatruns therein. Each resistor can be positioned along a respectivetransmission path so that the resistor is in series with an output of arespective current test pattern generator circuit and thus in serieswith a respective cryogenic DUT (e.g., when electrically coupled).

By way of example, the channel select circuit 136 can include circuitrythat can couple the first and second inputs 140 and 146 internally sothat the first and second inputs 140 and 146 are shorted therein. Insome instances, the first and second inputs 140 and 146 of the channelselect circuit 136 are shorted in response to the channel control signal134. For example, the channel select circuit 136 can include a switchthat can be positioned between the first and second inputs 140 and 146.The switch can be actuated and thus closed in response to the channelcontrol signal 134 to short the first and second inputs 140 and 146.When the switch is in an open state, the first and second inputs 140 and146 can be electrically coupled to respective first and second outputsof the channel select circuit 136 so that the cryogenic DUT 112 can beprovided test patterns. By shorting the first and second inputs 140 and146, a closed loop circuit 148 is formed in which the current testsignal 122 flows from the first output 118 of the current test patterngenerator circuit 104 through the short formed between the first andsecond inputs 140 and 146 through the resistor 144 back to the secondoutput 120 of the current test pattern generator circuit 104. A voltagecan be established across the resistor 144 based on the current testsignal 122.

The measuring circuit 126 includes a voltage sensing circuit 150 thatcan sense (e.g., measure) the voltage across the resistor 144 to providea sensed voltage signal 152. The voltage sensing circuit 150 can providethe sensed voltage signal 152 with a sufficient gain that raises thesensed voltage signal 152 above a noise floor of a voltage calculatorcircuit 154. This is because the sensed voltage signal 152 is in amicro-volt range or nano-volt range as the voltage across the resistor144 is based on a current that is in a micro-amperes or nano-amperesrange. The voltage calculator circuit 154 can be configured to samplethe sensed voltage signal 152 based on a sampling rate, which can be setby the controller 102. The voltage calculator circuit 154 can beconfigured to determine an average voltage based on the samples of thesensed voltage signal 152. The voltage calculator circuit 154 can beconfigured to output an average voltage signal 156 indicative of anaverage voltage of the sensed voltage signal 152. In some examples, thevoltage calculator circuit 154 can be configured to output an RMSvoltage based on the samples of the sensed voltage signal 152. Apolarity of the average voltage signal 156 can indicate in whichdirection a current flows through the resistor 144. For example, if thecurrent test signal 122 is provided from the second output 120 of thecurrent test pattern generator circuit 104 to the resistor 144, theaverage voltage signal 156 can have a negative polarity.

The controller 102 can be configured to receive the average voltagesignal 156. The impedance test logic 110 can cause the controller 102 todetermine transmission path characteristics based on the average voltagesignal 156 and an amount of current being outputted by the current testpattern generator circuit 104. The amount of current outputted by thecurrent test pattern generator circuit 104 can be determined based onthe sensed current test signal 132. For example, the processor 106 canexecute the impedance characteristic calculator 114 to determine thetransmission path characteristics for the first and second transmissionpaths 138 and 142.

Because a resistance of the resistor 144 and a voltage across theresistor 144 is known (based on the average voltage signal 156), and theamount of current outputted by the current test pattern generatorcircuit 104 is also known (based on the sensed current test signal 132),the impedance characteristic calculator 114 can be programmed todetermine first impedance characteristics representative of an overallimpedance of the first and second transmission paths 138 and 142. Inexamples wherein the current test signal 122 is the DC current testsignal the first determined impedance characteristics can characterizean overall DC impedance of the first and second transmission paths 138and 142. In examples wherein the current test signal 122 is the ACcurrent test signal the first determined impedance characteristics cancharacterize an overall AC impedance of the first and secondtransmission paths 138 and 142.

In some examples, the controller 102 can be configured to cause thechannel output circuit 124 to be in electrical connection with thecryogenic DUT 112. For example, the controller 102 can be configured tostop providing the channel control signal 134 to remove the shortbetween the first and second inputs 140 and 146 of the channel selectcircuit 136. The channel select circuit 136 in some instances can beconfigured to couple the first and second inputs 140 and 146 torespective outputs 158 and 160 of the channel select circuit 136 inresponse to not receiving the channel control signal 134. In otherexamples, the controller 102 can be configured to provide a different oranother channel control signal 134 to cause the channel select circuit136 to couple the first and second inputs 140 and 146 to respectiveoutputs 158 and 160 of the channel select circuit 136 therein. Once thefirst and second inputs 140 and 146 are coupled to respective outputs158 and 160 to which corresponding inputs of the cryogenic DUT 112 arecoupled, the cryogenic DUT 112 can be electrically coupled to the firstand second transmission paths 138 and 142.

Overall impedance characteristics for the first and second transmissionpaths 138 and 142 with the cryogenic DUT 112 in electrical connection(e.g., not electrically isolated) can be calculated in a same or similarmanner as described herein for calculating the first impedancecharacteristics for the first and second transmission paths 138 and 142.Thus, the controller 102 can be configured to determine second impedancecharacteristics representative of an overall impedance of the first andsecond transmission paths 138 and 142 during which the cryogenic DUT 112is electrically connected to the channel output circuit 124. In someinstances, the AC and DC impedance characteristics for the first andsecond transmission paths 138 and 142 can be determined in a same orsimilar manner as described herein. Because the cryogenic DUT 112 iselectrically coupled to the channel output circuit 124, impedancecharacteristics of one of the first and second transmission paths 138and 142 can change as the cryogenic DUT 112 can influence an impedanceof at least one of the first and second transmission paths 138 and 142.

The impedance characteristic calculator 114 can be programmed todetermine an impedance difference based on the first and seconddetermined impedance characteristics for the first and secondtransmission paths 138 and 142. The impedance difference can berepresentative of an impedance of the cryogenic DUT 112. For example,the impedance characteristic calculator 114 can be programmed tosubtract a second impedance from the first impedance to compute animpedance difference corresponding to the impedance of the cryogenic DUT112. In some examples, the impedance characteristic calculator 114 canbe programmed to compute a DUT current noise contribution and DUTvoltage drop. For example, the DUT voltage drop can be computed based ona difference in the voltage across the resistor 144 when the cryogenicDUT 112 is electrically isolated from the channel output circuit 124 andwhen the cryogenic DUT 112 is electrically coupled to the channel outputcircuit 124. In additional or alternative examples, the controller 102can be configured to determine the DUT current noise contribution byapplying a Fast Fourier Transform (FFT) based on the average voltagesignal 156. The controller 102 can be configured to compute the DUTcurrent noise contribution of the cryogenic DUT 112 based on adifference in the DUT current noise contribution when the cryogenic DUT112 is electrically coupled to the channel output circuit 124 andelectrically isolated from the channel output circuit 124.

Thus, the controller 102 can be configured to measure DUT electricalattributes, such as current (e.g., based on the resistance of theresistor 144 and the average voltage signal 156), a phase (e.g., basedon the current test signal 122 and the average voltage signal 156, andpower (e.g., consumed by the cryogenic DUT 112, such as based on thecurrent test signal 122 and the average voltage signal 156). Becausethese attributes can change depending on a DUTs performance, properoperating points of the DUT can be determined. Moreover, the measuredDUT electrical attributes can be used for diagnosing operational issuesof the cryogenic DUT 112.

Accordingly, the cryogenic DUT testing system 100 can accurately measureimpedance characteristics of the cryogenic DUT 112 (e.g., such as ACand/or DC impedance characteristics) by compensating for transmissionpath nonidealities and impedance mismatches between the first and secondtransmission paths 138 and 142.

FIG. 2 is an example of another cryogenic DUT testing system 200 thatcan be used for testing a performance and/or characteristics of acryogenic DUT (e.g., the cryogenic DUT 112, as shown in FIG. 1 ). Thus,in some examples, reference can be made to FIG. 1 in the example of FIG.2 . The system 200 can be implemented with a temperature compensationcircuit 204 that can be configured to compensate for temperature effectson a differential current test signal 206 provided to the cryogenic DUTduring cryogenic DUT testing. The temperature compensation circuit 204can correspond to a temperature compensation circuit as described inco-pending application entitled “Temperature Compensated Current Sourcefor Cryogenic Electronic Testing” (Attorney Docket No. NG(TCU)030771 USPRI) and filed concurrently with this application, which is incorporatedherein by reference in its entirety. The temperature compensationcircuit 204 can be used to mitigate temperature drift effects on thedifferential current test signal 206 and provide the cryogenic DUT withstable testing currents.

The system 200 includes a voltage source circuit 208. The voltage sourcecircuit 208 can be configured to provide a differentialtemperature-compensated voltage 210, which can be used to provide atemperature compensated differential current test signal (e.g., in someinstances corresponding to the differential current test signal 116, asshown in FIG. 1 ). In the example of FIG. 2 , the voltage source circuit208 includes the temperature compensation circuit 204, however, in otherexamples, the temperature compensation circuit 204 may be locatedoutside the voltage source circuit 208. For example, the temperaturecompensation circuit 204 can be implemented on a separate die and can becoupled to another die on which the voltage source circuit 208 can beimplemented. The temperature compensation circuit 204 can provide atemperature compensation signal 212 to a differential voltage generator214.

The differential voltage generator 214 can be configured to receive thetemperature compensation signal 212 and a reference signal 216 that canprovide a reference voltage. In some examples, the reference signal 216can correspond to the reference signal 128, as shown in FIG. 1 . Thedifferential voltage generator 214 can be configured to output thedifferential temperature compensated voltage 210 based on the referencesignal 216 and the temperature compensation signal 212. In someexamples, the reference signal 216 is a digital signal and thedifferential voltage generator 214 includes a digital to analog (DAC)converter. By way of example, the DAC converter can be a 20-bit DACconverter. In other examples, a DAC converter with a differentresolution type can be used. The system 200 can include a maincontroller 218 that can be configured to provide the reference signal216. The main controller 218 can correspond to the controller 102, asshown in FIG. 1 .

The system 200 can further include a current output circuit 220. Thecurrent output circuit 220 can include a VI converter 222. In someexamples, the differential voltage generator 214 and the VI converter222 can form a current test pattern generator circuit 224, such as thecurrent test pattern generator circuit 104, as shown in FIG. 1 . The VIconverter 222 can be configured to provide the differential current testsignal 206 based on the differential temperature compensated voltage210. The current output circuit 220 further includes a channel outputcircuit 226 that can be configured to provide the differential currenttest signal 206 as a temperature compensated differential test currentto the cryogenic DUT.

In the example of FIG. 2 , a single channel is shown for providing adifferential test current for testing a single cryogenic DUT. Thus,while the example of FIG. 2 is described with respect to a singlechannel, in other examples, any number of channels (e.g., twelve (12)channels) can be used for testing a corresponding number of cryogenicDUTs. The cryogenic DUTs can be placed in a container, cooler, fridge,or chamber that is configured to support a cryogenic environment. Eachchannel can be enabled or activated during cryogenic testing of thecorresponding cryogenic DUT by a channel controller 228 to provide arespective differential test current to the corresponding cryogenic DUT.

By way of example, the current test pattern generator circuit 224 has afirst output 230 and a second output 232. In some instances, the firstoutput 230 provides a current test signal 234 and the second output 232receives the current test signal 122. The current test signal 234 can besimilar to the current test signal 122, as shown in FIG. 1 . Thus, thefirst output 230 can output the current test signal 234 and the secondoutput 232 can pull the current test signal 234. Because the system 200is for testing cryogenic DUTs, the current test signal 234 can bereferred to as a high-precision test current. In some instances, thecurrent test signal 234 is an AC current test signal and in otherinstances is a DC current test signal. The AC and DC current testsignals can be used to determine AC and DC impedance characteristics oftransmission paths and the cryogenic DUT such as described herein.

In further examples, the current output circuit 220 includes adifferential amplifier 236 and an average voltage calculator circuit238. The differential amplifier 236 can correspond to the voltagesensing circuit 150, as shown in FIG. 1 , and the average voltagecalculator circuit 238 can correspond to the voltage calculator circuit154, as shown in FIG. 1 . In some instances, the differential amplifier236 and the average voltage calculator circuit 238 can define ameasuring circuit 240, such as the measuring circuit 126, as shown inFIG. 1 .

Continuing with the example of FIG. 2 , the main controller 218 can beconfigured to execute impedance test logic (e.g., the impedance testlogic 110, as shown in FIG. 1 ) for testing impedance characteristics ofthe cryogenic DUT to provide the reference signal 216. The current testpattern generator circuit 224 can provide the differential current testsignal 206, which includes the current test signal 234, based on thereference signal 216. The system 200 includes a current sensing circuit242 to measure an amount of current being outputted by the current testpattern generator circuit 224. The current sensing circuit 242 can sense(e.g., sample) the current test signal 234 to provide a sensed currenttest signal 244. The channel controller 228 can receive and communicatethe sensed current test signal 244 to the main controller 218.

The main controller 218 can be configured to provide a channel controlsignal 246. The channel control signal 246 can correspond to the channelcontrol signal 134, as shown in FIG. 1 . The main controller 218 canprovide the channel control signal 246 to the channel controller 228.The channel control signal 246 can control an electrical coupling of thechannel output circuit 226 to the cryogenic DUT. The channel controlsignal 246 can be provided by the channel controller 228 to a channelselect circuit 248 to electrically isolate the channel output circuit226 from the cryogenic DUT. By electrically isolating the channel outputcircuit 226 from the cryogenic DUT impedance characteristics oftransmission paths can be estimated or determined according to theexamples described herein.

In some instances, the channel select circuit 248 is configured in afirst state (e.g., DUT isolating state) in which the channel outputcircuit 226 is electrically isolated from the cryogenic DUT. In exampleswherein the channel select circuit 248 is in the first state, thechannel controller 228 can provide the channel control signal 246 toswitch the channel select circuit 248 to a second state, therebyelectrically coupling the channel output circuit 124 to the cryogenicDUT. In some instances, the channel select circuit 248 includes a relay250, such as a SPDT relay.

In the example of FIG. 2 , a first transmission path 252 can extend fromthe first output 230 of the current test pattern generator circuit 224through a first resistor 254) to a first input 256 of the relay 250. Asecond transmission path 258 can extend from the second output 232 ofthe current test pattern generator circuit 224 through a second resistor260 to a second input 262 of the relay 250, as shown in FIG. 2 . Thefirst and second resistors 254 and 260 can have respective resistances(labelled as “R1” and “R2” in the example of FIG. 2 ). The first andsecond resistors 254 and 260 can have complementary resistances. In anexample, the relay 250 is a SPDT relay.

In some instances, the first and second inputs 256 and 262 of the relay250 are shorted to form a short 263 in response to the channel controlsignal 246. By shorting the first and second inputs 256 and 262, thecurrent test signal 234 flows from the first output 230 of the currenttest pattern generator circuit 224 through the first resistor 254 andthe short 263 formed between the first and second inputs 256 and 262through the second resistor 260 to the second output 232 of the currenttest pattern generator circuit 224. A respective voltage can beestablished across the first and second resistors 254 and 262 based onthe current test signal 234. The differential amplifier 236 can beconfigured to sense (e.g., measure) the voltage across the firstresistor 254 to provide a sensed voltage signal 264. While the exampleof FIG. 2 illustrates the voltage across the first resistor 254 beingsensed in other examples the voltage across the second resistor 260 canbe sensed to provide the sensed voltage signal 264.

The differential amplifier 236 can provide the sensed voltage signal 264with a sufficient gain that raises the differential amplifier 236 abovea noise floor of the average voltage calculator circuit 238. This isbecause the sensed voltage signal 264 is in a micro-volt range ornano-volt range as the voltage across the first resistor 254 is based ona current that is in a micro-amperes or nano-amperes range. The averagevoltage calculator circuit 238 can be configured to sample the sensedvoltage signal 264 based on a sampling rate, which can be set by thechannel controller 228. The average voltage calculator circuit 238 canbe configured to determine an average voltage based on the samples ofthe sensed voltage signal 264. The average voltage calculator circuit238 can be configured to output an average voltage signal 266 indicativeof an average voltage of the sensed voltage signal 264. A polarity ofthe average voltage signal 266 can indicate in which direction currentflows through the first resistor 254. For example, if the current testsignal 234 is provided from the second output 232 of the current testpattern generator circuit 224 and thus flows in an opposite direction ifprovided from the first output 230, the average voltage signal 266 canhave a negative polarity.

The channel controller 228 can be configured to provide the averagevoltage signal 266 to the main controller 218. The main controller 218can be configured to determine transmission path characteristics basedon the average voltage signal 266 and an amount of current outputted bythe current test pattern generator circuit 224. The amount of currentoutputted by the current test pattern generator circuit 224 can bedetermined based on the sensed current test signal 244. For example, themain controller 218 can execute a characteristic calculator (e.g., thecharacteristic calculator 114, as shown in FIG. 1 ) which can beprogrammed to determine the transmission path characteristics for thefirst and second transmission paths 252 and 258.

Because a resistance of the first resistor 254 and a voltage across thefirst resistor 254 is known (based on the average voltage signal 266),and the amount of current outputted by the current test patterngenerator circuit 224 is also known (based on the sensed current testsignal 244), the main controller 218 can be configured to determinefirst impedance characteristics representative of an overall impedanceof the first and second transmission paths 252 and 258. In exampleswherein the current test signal 234 is the DC current test signal thefirst determined impedance characteristics can characterize an overallDC impedance of the first and second transmission paths 252 and 258. Inexamples wherein the current test signal 234 is the AC current testsignal the first determined impedance characteristics can characterizean overall AC impedance of the first and second transmission paths 252and 258.

In some examples, the main controller 218 can be configured to cause thechannel output circuit 226 to be in electrical connection with thecryogenic DUT 112. For example, the main controller 218 can communicatewith the channel controller 228 to cause the channel controller 28 tostop providing the channel control signal 246 to remove the short 233between the first and second inputs 256 and 262 of the relay 250.Overall impedance characteristics for the first and second transmissionpaths 252 and 258 with the DUT in electrical connection (e.g., notelectrically isolated) can be calculated in a same or similar manner asdescribed herein for calculating the first impedance characteristics forthe first and second transmission paths 252 and 258. Thus, the maincontroller 218 can be configured to determine second impedancecharacteristics representative of an overall impedance of the first andsecond transmission paths 252 and 258 during which the cryogenic DUT iselectrically connected to the channel output circuit 226. In someinstances, the AC and DC impedance characteristics for the first andsecond transmission paths 252 and 258 can be determined in a same orsimilar manner as described herein. Because the cryogenic DUT iselectrically coupled to the channel output circuit 226, impedancecharacteristics of one of the first and second transmission paths 252and 258 can change as the cryogenic DUT can influence an impedance of atleast one of the first and second transmission paths 252 and 258.

The main controller 218 can be configured to determine an impedancedifference based on the first and second determined impedancecharacteristics for the first and second transmission paths 252 and 258.The impedance difference can be representative of an impedance of thecryogenic DUT. For example, the main controller 218 can be configured tosubtract a second impedance from a first impedance to compute animpedance difference corresponding to the impedance of the cryogenicDUT.

Accordingly, the cryogenic DUT testing system 200 can accurately measureimpedance characteristics of the cryogenic DUT (e.g., such as AC and/orDC impedance characteristics) by compensating for transmission pathnonidealities and impedance mismatches between the first and secondtransmission paths 252 and 258.

FIG. 3 is an example of a current test pattern generator circuit 300.The current test pattern generator circuit 300 can be the current testpattern generator circuit 104, as shown in FIG. 1 , or the current testpattern generator circuit 224, as shown in FIG. 2 . Thus, in someexamples, reference can be made to FIGS. 1-2 in the example of FIG. 3 .The current test pattern generator circuit 300 includes asingle-ended-to-differential converter 302, a DAC controller 304, and atemperature compensation circuit 306, which can correspond to thetemperature compensation circuit 204, as shown in FIG. 2 . Thesingle-ended-to-differential converter 302 and the DAC controller 304can define or form the differential voltage generator 214, as shown inFIG. 2 . Thus, the DAC controller 304 can be configured to receive areference signal 308. The reference signal 308 can be representative ofa digital signal and characterize a reference voltage to be provided bythe DAC controller 304 to the single-ended-to-differential converter302. In some instances, the reference signal 308 corresponds to thereference signal 128, as shown in FIG. 1 . The DAC controller 304 can beconfigured to provide an intermediate reference voltage 310 to thesingle-ended-to-differential converter 302 based on the reference signal308. The temperature compensation circuit 306 can correspond to thetemperature compensation circuit 204, as shown in FIG. 2 , and can beconfigured to provide a temperature compensation signal 312. Thetemperature compensation signal 312 can be the temperature compensationsignal 212, as shown in FIG. 2 . The single-ended-to-differentialconverter 302 can be configured to output a differential temperaturecompensated voltage 314 based on a difference between the intermediatereference voltage 310 and the temperature compensation signal 312. Thedifferential temperature compensated voltage 314 can correspond to thedifferential temperature compensated voltage 210, as shown in FIG. 2 .

In view of the foregoing structural and functional features describedabove, example methods will be better appreciated with reference toFIGS. 4-5 . While, for purposes of simplicity of explanation, theexample methods of FIGS. 4-5 are shown and described as executingserially, it is to be understood and appreciated that the presentexamples are not limited by the illustrated order, as some actions couldin other examples occur in different orders, multiple times and/orconcurrently from that shown and described herein. Moreover, it is notnecessary that all described actions be performed to implement eachmethod below.

FIG. 4 is an example of a method 400 for determining impedancecharacteristics of a cryogenic DUT such as the cryogenic DUT 112, asshown in FIG. 1 . Thus, in some examples, reference can be made to FIGS.1-3 in the example of FIG. 4 . The method 400 can begin at 402 byelectrically isolating a channel output circuit (e.g., the channeloutput circuit 124, as shown in FIG. 1 ) from the cryogenic DUT. At 404,providing a first current test signal (e.g., the current test signal122, as shown in FIG. 1 , or the current test signal 234, as shown inFIG. 2 ) along a transmission path (e.g., the first and secondtransmission paths 138 and 142, as shown in FIG. 1 ) such that the firstcurrent test signal flows through at least one resistor (the resistor144, as shown in FIG. 1 ) located in the transmission path.

At 406, measuring a first voltage across the at least one resistor basedon the first current test signal. At 408, determining a first averagevoltage based on measurements of the first voltage across the resistor.At 410, determining first impedance characteristics of the transmissionpath based on the first average voltage and an amount of current beingprovided by the first current test signal. At 412, electrically couplingthe channel output circuit to the cryogenic DUT such that the cryogenicDUT is part of the transmission path. At 414, providing a second currenttest signal (e.g., the current test signal 122, as shown in FIG. 1 , orthe current test signal 234, as shown in FIG. 2 ) along the transmissionpath such that the second current test signal flows through thecryogenic DUT and the at least one resistor. At 416, measuring a secondvoltage across the at least one resistor based on the second currenttest signal. At 418, determining a second average voltage based onmeasurements of the second voltage across the at least one resistor. At420, determining second impedance characteristics of the transmissionpath based on the second average voltage and an amount of current beingprovided by the second current test signal. At 422, determiningimpedance characteristics of the cryogenic DUT based on the determinedfirst and second impedance characteristic of the transmission path.

FIG. 5 is an example of another method 500 for determining impedancecharacteristics of a cryogenic DUT, such as the cryogenic DUT 112, asshown in FIG. 1 . The method 500 can be implemented by a controller,such as the controller 102, as shown in FIG. 1 , or the main controller218, as shown in FIG. 2 . Thus, in some examples, reference can be madeto FIGS. 1-3 in the example of FIG. 5 . At 502, causing a channel outputcircuit (e.g., the channel output circuit 124, as shown in FIG. 1 ) tobe electrically isolated from the cryogenic DUT. At 504, controlling atest pattern generator circuit (e.g., the current test pattern generatorcircuit 104, as shown in FIG. 1 ) to provide a first current test signal(e.g., the current test signal 122, as shown in FIG. 1 , or the currenttest signal 234, as shown in FIG. 2 ) along a transmission path (e.g.,the first and second transmission paths 138 and 142, as shown in FIG. 1) so that the first current test signal flows through at least oneresistor (the resistor 144, as shown in FIG. 1 ) located in thetransmission path to establish a first voltage across the at least oneresistor. At 506, receiving a first determined average voltage based onmeasurements of the first voltage across the at least one resistor. At508, determining first impedance characteristics of the transmissionpath based on the determined first average voltage and an amount ofcurrent being provided by the first current test signal.

At 510, causing the channel output circuit to be electrically coupled tothe cryogenic DUT such that the cryogenic DUT is part of thetransmission path. At 512, controlling the test pattern generatorcircuit to provide a second current test signal along the transmissionpath so that the second current test signal flows through the cryogenicDUT and the at least one resistor to establish a first voltage acrossthe at least one resistor. At 514, receiving a second determined averagevoltage based on measurements of the second voltage across the at leastone resistor. At 516, determining second impedance characteristics ofthe transmission path based on the second average voltage and an amountof current being provided by the second current test signal. At 518,determining impedance characteristics of the cryogenic DUT based on thefirst and second impedance characteristic of the transmission path.

What has been described above are examples. It is, of course, notpossible to describe every conceivable combination of components ormethodologies, but one of ordinary skill in the art will recognize thatmany further combinations and permutations are possible. Accordingly,the disclosure is intended to embrace all such alterations,modifications, and variations that fall within the scope of thisapplication, including the appended claims. As used herein, the term“includes” means includes but not limited to, the term “including” meansincluding but not limited to. The term “based on” means based at leastin part on. Additionally, where the disclosure or claims recite “a,”“an,” “a first,” or “another” element, or the equivalent thereof, itshould be interpreted to include one or more than one such element,neither requiring nor excluding two or more such elements.

What is claimed is:
 1. A system comprising: a channel select circuitconfigured in a first state to electrically isolate a channel outputcircuit from a cryogenic device under test (DUT) and in a second stateto electrically couple the channel output circuit to the cryogenic DUT;at least one resistor positioned along a transmission path that couplesa pattern generator circuit to the channel output circuit that includesthe channel select circuit; a controller configured to: cause respectivetest current signals to be provided by the pattern generator circuitalong the transmission path when the channel select circuit is inrespective first and second states to establish respective first andsecond voltages across the at least one resistor; determine first andsecond impedance characteristics of the transmission path based on theestablished respective first and second voltages and an amount ofcurrent being provided by respective test current signals; and determinean impedance of the cryogenic DUT based on the first and seconddetermined impedance characteristics of the transmission path.
 2. Thesystem of claim 1, wherein the transmission path has a first impedancewhen the channel select circuit is in the first state, and thetransmission path has a second impedance when the channel select circuitis in the second state, wherein the cryogenic changes an impedance ofthe transmission path from the first impedance to the second impedancein response to being electrically coupled to the transmission path whenthe channel select circuit is in the second state.
 3. The system ofclaim 2, further comprising a measuring circuit configured to measurethe first voltage across the at least one resistor when the channeloutput circuit is in the first state and measure the second voltageacross the at least one resistor when the channel output circuit is inthe second state.
 4. The system of claim 3, wherein the measuringcircuit comprises a voltage sensing circuit to measure the first andsecond voltage across the at least one resistor and a voltage calculatorcircuit configured to determine a first average voltage based onmeasurements of the first voltage across the at least one resistor, anddetermine a second average voltage based on measurements of the secondvoltage across the at least one resistor.
 5. The system of claim 4,wherein respective test current signals includes first and secondcurrent test signals, the controller being configured to determine thefirst impedance characteristic of the transmission path based on thefirst average voltage and an amount of current being provided by thefirst current test signal, and determine the second impedancecharacteristic of the transmission path based on the second averagevoltage and an amount of current being provided by the second currenttest signal.
 6. The system of claim 5, wherein the controller isconfigured to subtract the second impedance characteristics of thetransmission path from the first impedance characteristics of thetransmission path to compute an impedance difference representative ofthe impedance of the cryogenic DUT.
 7. The system of claim 6, furthercomprising a current sensing circuit configured to measure the amount ofcurrent being provided as the first current test signal and the secondcurrent test signal, respectively.
 8. The system of claim 7, wherein thetransmission path comprises a first transmission path that extends froma first output of the pattern generator circuit to a first input of thechannel select circuit and a second transmission path that extends froma second output of the pattern generator circuit to a second input ofthe channel select circuit.
 9. The system of claim 8, wherein the atleast one resistor is positioned along a respective one of the first andsecond transmission paths, and the system further comprises anotherresistor positioned along one of the remaining first and secondtransmission paths.
 10. The system of claim 9, wherein the channelselect circuit comprises a relay having first and second inputscorresponding to the first and second inputs of the channel selectcircuit, the relay be configured to short the first and second inputs toform a closed loop circuit between the pattern generator circuit and thechannel select circuit in response to a channel control signal providedby the controller to set the channel select circuit to the first state.11. A circuit comprising: a measuring circuit configured to: measure afirst voltage established across a resistor along a transmission pathbased on a first current signal, wherein the first current test signalis provided along the transmission path when a channel output circuit isisolated from a cryogenic device under test (DUT), wherein thetransmission path couples a pattern generator circuit for generatingcurrent signals to a channel output circuit; measure a second voltageestablished across the resistor based on a second current signal,wherein the second current test signal is provided along thetransmission path when the channel output circuit is electricallycoupled to cryogenic DUT, determine first and second average voltagesbased on the measurements of the first and second voltages,respectively; a controller configured to: determine first and secondimpedance characteristics of the transmission path based on respectivefirst and second average voltages and an amount of current beingprovided by respective first and second current test signals; anddetermine an impedance of the cryogenic DUT based on the first andsecond determined impedance characteristics of the transmission path.12. The system of claim 11, wherein the transmission path has a firstimpedance when the channel output circuit is isolated from the cryogenicDUT, and the transmission path has a second impedance when the channeloutput circuit is electrically coupled to the cryogenic DUT, wherein thecryogenic DUT changes an impedance of the transmission path from thefirst impedance to the second impedance in response to beingelectrically coupled to the transmission path by the channel outputcircuit.
 13. The system of claim 12, wherein the measuring circuitcomprises a voltage sensing circuit to measure the first and secondvoltages across the resistor and a voltage calculator circuit configuredto determine the first and second average voltages based on respectivemeasurements of the first and second voltages.
 14. The system of claim13, wherein the controller is configured to subtract the secondimpedance characteristics of the transmission path from the firstimpedance characteristics of the transmission path to compute animpedance difference representative of the impedance of the cryogenicDUT.
 15. The system of claim 14, wherein the transmission path comprisesa first transmission path that extends from a first output of thepattern generator circuit to a first input of the channel select circuitand a second transmission path that extends from a second output of thepattern generator circuit to a second input of the channel selectcircuit.
 16. The system of claim 15, wherein the resistor is positionedalong a respective one of the first and second transmission paths, andthe system further comprises another resistor positioned along one ofthe remaining first and second transmission paths.
 17. The system ofclaim 16, wherein the channel select circuit comprises a relay havingfirst and second inputs corresponding to the first and second inputs ofthe channel select circuit, the relay be configured to short the firstand second inputs to form a closed loop circuit between the patterngenerator circuit and the channel select circuit in response to achannel control signal provided by the controller.
 18. A methodcomprising: causing a channel output circuit to be electrically isolatedfrom a cryogenic device under test (DUT); controlling a test patterngenerator circuit to provide a first current test signal along atransmission path so that the first current test signal flows through atleast one resistor located in the transmission path to establish a firstvoltage across the at least one resistor; receiving a first determinedaverage voltage based on measurements of the first voltage across the atleast one resistor; determining first impedance characteristics of thetransmission path based on the determined first average voltage and anamount of current being provided by the first current test signal;causing the channel output circuit to be electrically coupled to thecryogenic DUT such that the cryogenic DUT is part of the transmissionpath in response to determining the first impedance characteristics ofthe transmission path; controlling the test pattern generator circuit toprovide a second current test signal along the transmission path so thatthe second current test signal flows through the cryogenic DUT and theat least one resistor to establish a first voltage across the at leastone resistor; receiving a second determined average voltage based onmeasurements of the second voltage across the at least one resistor;determining second impedance characteristics of the transmission pathbased on the second average voltage and an amount of current beingprovided by the second current test signal; and determining impedancecharacteristics of the cryogenic DUT based on the first and secondimpedance characteristic of the transmission path.
 19. The method ofclaim 18, wherein causing the channel output circuit to be electricallycoupled to the cryogenic DUT such that the cryogenic DUT is part of thetransmission path comprises providing a channel control signal for achannel select circuit of the channel select circuit to cause thechannel select circuit to switch from a first state to a second state,wherein in the first state the channel output circuit is electricallyisolated from the cryogenic DUT and in the second state channel outputcircuit is electrically coupled to the cryogenic DUT.
 20. The method ofclaim 19, wherein the determining impedance characteristics of thecryogenic DUT based on the first and second impedance characteristic ofthe transmission path comprises subtracting the second impedancecharacteristics of the transmission path from the first impedancecharacteristics of the transmission path to compute an impedancedifference representative of the impedance of the cryogenic DUT.